1. Field of the Invention
The present invention relates to an apparatus and method for forming a film, more particularly to an apparatus and method for forming a high temperature superconducting film on a long tape substrate at high speeds suitable for large scale production.
2. Description of the Prior Art
A superconductor can carry high current without power loss because it has a property that the resistance is reduced to zero below a critical temperature. Therefore many studies have been made to use high temperature superconductors in applications such as power transmission lines and high field electromagnets. If the studies are commercially practiced, a big demand will be created in the marketplace of the various electromagnetic power devices. For this reason, most studies have been focused on manufacturing long flexible high temperature superconducting wires having high critical current densities in a large scale production.
However, recently the manufacture of high temperature superconducting tapes is also actively studied. Generally, there are two types of high temperature superconducting tapes according to the superconducting materials: Bismuth based ("Bi-type") high temperature superconducting material; and Yttrium based ("Y-type") high temperature superconducting material. The Bi-type material has proven to be easy to process since it retains good mechanical properties. The Y-type material is difficult to process but has an advantage that it is suitable for use in a high magnetic field. Bi-type high temperature superconducting materials have been studied intensively all over the world, but more recently Y-type high temperature superconducting materials are actively studied.
Unlike Bi-type high temperature superconducting materials (for example, Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x), Y-type high temperature superconducting materials including Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x ("YBCO") have poor mechanical properties, so that Y-type superconducting tapes should be manufactured by methods different from those used in manufacturing Bi-type superconductor tapes. One of the methods of manufacturing YBCO superconductor tapes is to form a YBCO film on a tape substrate.
The tape substrate may be supplied in any conventional manner, shown in FIG. 6 as being unwound from a first spool 52. FIG. 6 is a schematic representation of a conventional "reel-to-reel" apparatus which is used in manufacturing a YBCO superconductor tape. Referring to FIG. 6, the first spool 52 continuously feeds the tape substrate toward deposition area 51 where the YBCO film is deposited. And then, the finished product, comprising the tape substrate and the YBCO film deposited thereon, is wound around a second spool 50.
FIG. 7 shows the deposition process performed by the "reel-to-reel" apparatus shown in FIG. 6. Referring to FIG. 7, a tape substrate is disposed to pass in proximity to vapors of superconduting materials when the tape substrate is moved horizontally in a direction of the arrow. The vapors are deposited on the tape and form a film. The portion A2 is deposited after the portion A1 has achieved its final deposition thickness. Thereafter, the portion A3 is deposited. That is, a YBCO superconducting film is produced by forming partially deposited portions continuously. In this process, at the locations, X (the edge of the vapor column) and Y (the center of the vapor column), the densities of vapor are so different that the parts of the film, being formed at that instance, at the two locations have different characteristics due to the different deposition rate. The lower layer in a part of the tape is deposited first when that part passes the location X, and then the upper layer of that part is deposited next when it passes the location Y later. Consequently, the film formed so is nonuniform in depth. The process has therefore an disadvantage that the deposited film is basically nonuniform.
Moreover, the deposition speed of this process has a serious limit since the deposition depends upon the moving speed of the tape substrate, which is determined by the size of vapor column times the deposition rate, where the former is few centimeter (same as the mean free path of the oxygen partial pressure) and the later is few nanometer thickness growth per minute (required for the epitaxial growth). The process is therefore not believed to be well suited for large scale production of superconductor tapes.
There can not be an over-emphasis of the importance of the film uniformity and the fabrication speed by the following reasons. If there is an insufficient superconducting quality at just one point over the whole length of few hundred meters long tape, that tiny defect will destroy the full super-current which flows along the whole length. The tape is so long that the fabrication speed per unit length should be enough fast.
Therefore, it is an object of the present invention to provide an apparatus and method, wherein a highly uniform high temperature superconducting film is formed on a tape substrate.
It is other object of the present invention to provide an apparatus and method for producing high temperature superconductor tapes at speeds suitable for large scale production.